1. Field of the Invention
This invention relates to semiconductor chip packaging technology and specifically to the integration of a heatsink element directly into the body of a semiconductor chip package.
2. Related Art
Packaging is one of the final steps in the process of manufacturing semiconductor chips. In packaging, a fabricated semiconductor chip is mounted within a protective housing. At the present moment, the art of semiconductor chip technology has evolved far more quickly than the integrally related technology of packaging the semiconductor chips. The packaging requirements of the newer, smaller, more powerful semiconductor chips are quickly progressing beyond the capabilities of traditional packaging technology and the conventional materials and designs presently utilized are quickly becoming obsolete. The packaging of new semiconductors needs new configurations to accommodate increasing numbers of electrical interconnections, constraints due to decreasing size, and, most importantly, exponentially growing heat transfer needs. The need to adequately transfer heat out of increasingly smaller semiconductor packages has spawned significant interest in the development of new packaging materials and more thermally efficient configurations.
Currently, semiconductor packaging utilizes the art of attaching an external heatsink to improve the heat transfer characteristics of many chip packages which do not adequately transfer heat away from the semiconductor chip. However, with the increasing heat transfer needs of new semiconductor chips the addition of an external heatsink is no longer thermally efficient.
In heat transfer technology, the two most significant modes of heat transfer are heat conduction and heat convection. Heat conduction is the transfer of heat either, through a single solid medium, or from one solid medium to another adjacent solid medium. The transfer of heat through the medium or between mediums is based on a temperature differential, i.e. heat flowing from hot to cold, until an equilibrium is reached. Heat convection is the transfer of heat away from a hot, solid medium to a cooler ambient body of air which has a generally constant temperature. The heat is convected away by warm air currents created by the warming of the air in close proximity to the hot, solid medium. Both of these heat transfer modes are paramount in evaluating the heat transfer characteristics of semiconductor packages.
The heat dissipating characteristics of semiconductor packages are measured by a network of heat transfer pathways through which heat must flow. The heat must be conducted from the chip through various thermal pathways to reach an outer surface of the package exposed to an ambient air body and then be convected away to that ambient air body.
Every packaging material has its own unique thermal characteristics. One of these characteristics is known as thermal conductivity. The thermal conductivity of a material determines the amount of heat that can be conducted through and away from that material. Some materials have high thermal conductivity, such as metals, and other materials have low thermal conductivity, such as rubber or glass. Materials having low thermal conductivity are known as thermal insulators. The most common semiconductor packaging materials, such as glass and glass ceramic composites, which allow for ease of manufacturing, have low thermal conductivities which hinder the dissipation of heat.
Depending on the construction of the semiconductor package, a package may have only a few materials or may have a multitude of various materials of varying thermal conductivity through which heat must be conducted to reach the ambient air. Because thermal conductivities are coupled together where materials meet, similar to electrical resistances, the more materials present between the source of heat generation and the ambient air mass, the more restricted the heat conduction will be.
Once heat from the semiconductor chip has been conducted through the various materials of the package and reaches the surface of the package it must be convected away to an ambient air mass. This process involves another thermal pathway.
As noted previously, heat transfer from the external surfaces of semiconductor packages is normally handled by the external addition of specially configured heatsinks which are mounted over the areas of greatest heat generation. This allows the heat to be conducted from the surface of the package, which usually has a low thermal conductivity, into the heatsink, which preferably has a high thermal conductivity and a large surface area, and then be convected away to the ambient air mass. The shape of the heatsinks are configured to allow for a large surface area which increases heat convection away from the package. In previous applications, where the heat generation of semiconductor chips was moderate the addition of an external heatsink worked well. The additional heat transfer capability created by externally bonding the heatsink to the package was sufficient to transfer any heat generated by the chip.
However, as heat generation has increased exponentially in the newer, smaller semiconductors, the effectiveness of externally mounted heatsinks has decreased. The thermal pathways of the package and heatsink can no longer adequately transfer the heat generated and can subsequently cause malfunctions within the semiconductors.
A major heat transfer problem common to many semiconductor packages is the configuration of the package and the mounting location of the semiconductor chip. Advantageously locating the chip can eliminate several thermal boundaries and increase heat transfer. Because the chip is routinely attached to an internal substrate, usually ceramic alumina, the heat generated from the chip must pass through that substrate before it reaches the heatsink mounted on the outer surface of the package. The substrate, along with the bonding material used to attach the external heatsink, add thermal barriers to the package and limit the maximum heat transfer capability of the package.
Accordingly, the purpose of the present invention is to overcome the packaging problems associated with the transfer of heat out of semiconductor packages.